The present invention relates to a process for producing poreless plate-like magnetite particles which have a large plate ratio (average particle diameter/thickness) and are discrete each other, directly from an aqueous solution and the plate-like magnetite particles.
The plate-like magnetite particles according to the present invention are chiefly used for electromagnetic wave absorbers, shields, anticorrosive coatings, toners, damper, sound insulator and magnetic recording material.
Plate-like magnetite particles are expected to be used in various technical fields by utilizing various properties such as the shape and the magnetic characteristics thereof.
This fact is described in Japanese Patent Publication No. 63-41853 (1988): "There is a possibility of using lamella magnetic iron oxide particles for a magnetic recording material such as magnetic tape, card and disk", and "Hexagonal lamella iron oxide particles having a magnetite or maghemite crystalline structure are also used for other purposes. . . . In a coating material, very remarkable parallel orientation of each particle is produced with respect to each other. Therefore, . . . it is possible to obtain very high packing density, and as a result, for example, the anticorrosive effect and the screening effect on electromagnetic interference increase, and the conductivity is enhanced". In Japanese Patent Application Laid-Open (KOKAI) No. 61-138959 (1986): "It has been found that when small plate-like or scale-like magnetizable particles are used as a magnetic component of one-component toner, it is scarcely by the intrinsic color of the magnetic component itself but mainly by the coloring agent having a strong color which is added thereto that the color of the toner is decided", and "It is possible to immediately convert the state of these particles into a good dispersed state in a binder or a binder solution merely by stirring them without consuming any special classifying energy".
Magnetite particles are generally dispersed in and mixed with a vehicle so as to be used as a coating, or are kneaded with and dispersed in a resin and molded so as to be used as a molded product.
In dispersing and mixing magnetite particles in and with a vehicle, or kneading and dispersing magnetite particles with and in a resin, the particles are required to have as a good dispersibility as possible in order to improve the properties and the working efficiency. For this purpose, magnetite particles are required not only to be in the form of a plate but also to have as large a plate ratio (average particle diameter/thickness) as possible, and be discrete each other and poreless.
As a process for producing plate-like magnetite particles, for example, a process of producing plate-like hematite particles from an aqueous solution by autoclaving an alkaline suspension containing ferric hydroxide or goethite and heat-treating the plate-like hematite particles in a reducing gas [refer to Japanese Patent Application Laid-Open (KOKAI) No. 51-28700 (1976), Japanese Patent Publication No. 63-41853 (1988)], a process of producing goethite particles by rapidly oxidizing an alkaline suspension containing ferrous hydroxide with a strong oxidizing agent or a process of producing goethite particles by reacting ferric iron with an alkali in an aqueous medium to produce ferric hydroxide and autoclaving the said ferric hydroxide, thereby forming plate-like goethite particles from an aqueous solution, and the heat-treating the goethite particles in a reducing gas after dehydrating the plate-like goethite particles under heating, if necessary [refer to Japanese Patent Application Laid-Open (KOKAI) Nos. 61-266311 (1986) and 55-104923 (1980)], and a process of producing plate-like magnetite particles directly from an aqueous solution refer to Japanese Patent Application Laid-Open (KOKAI) No. 62- 34141).
Poreless plate-like magnetite particles which have as large a plate ratio (average particle diameter/thickness) as possible and which are discrete each other, are now strongly demanded, but particles having satisfactory properties have not been obtained by the above-described known methods.
In a process of producing plate-like hematite particles from an aqueous solution and heat-treating these particles, a sintering of particle and between particles occurs in the heat-treating process, which makes it difficult to disperse the particles in a vehicle or a resin.
In the case of producing plate-like goethite particles from an aqueous solution and after, if necessary, dehydrating the particles under heating, the obtained plate-like magnetite particles have many pores on the particle surface and in the inside of particle as a result of dehydration in the goethite crystal particles, and a sintering of particle and between particles occurs, when they are heated. If such porous magnetite particles are dispersed in a vehicle or a resin, other fine particles are attracted to the portion at which surface magnetic poles are generated. As a result, a multiplicity of particles agglomerate and a considerably large agglomerated lumps are produced, which make dispersion difficult.
By a process of producing plate-like magnetite particles directly from an aqueous solution, the particles obtained are discrete from each other and poreless, but the plate ratio (average particle diameter/thickness) is at most about 9:1, and particles having a large plate ratio have not been obtained.
As a result of the studies undertaken by the present inventors, it has been found that the particles obtained by oxidizing a suspension containing FeCO.sub.3 obtained by reacting an aqueous ferrous salt solution and an aqueous alkali carbonate solution by passing an oxygen-containing gas therethrough, wherein the suspension is obtained by reacting a ferrous salt solution containing 0.2 to 5.0 mol % of a ferric salt, calculated as Fe(III), based on Fe(II) and an aqueous alkali carbonate the amount of which is so controlled that the equivalent ratio of an alkali carbonate to the Fe(II) and Fe(III) in the aqueous ferrous salt solution is not less than 1 and the following general formula is satisfied: ##EQU2## and if necessary, 0.01 to 8.0 mol % of a water-soluble organic compound having a --COONa or --COOK group based on the Fe(II) and Fe(III) are added to any of the aqueous ferrous salt solution, the aqueous alkali carbonate solution and the suspension containing FeCO.sub.3 before being oxidized by passing the oxygen-containing gas therethrough, are poreless plate-like magnetite particles which have as large a plate ratio (average particle diameter/thickness) as possible and which are discrete each other. On the basis of this finding, the present invention has been achieved.